1. Field of the Invention
The present invention relates to an oil return structure for an internal combustion engine.
2. Description of the Related Art
In a known art, an oil pan is located at the bottom of the crankcase of an internal combustion engine. The oil pan holds oil (lubricating oil) to lubricate and cool moving (sliding) engine parts. Oil held in the oil pan is delivered to the respective engine parts by the oil pump when the engine runs. After lubricating and cooling these engine parts, oil returns to and is collected in the oil pan. For example, oil is delivered to the engine cylinder head, lubricates the valve driving system, and then drops back down into the oil pan.
According to the known art, the engine cylinder head and cylinder block have an oil return hole. Oil returns to the oil pan through the oil return hole, after being delivered to the cylinder head and lubricating the valve driving system. The oil return hole serves as an oil return passage to communicate the cam chamber, located at the upper side of the engine, with the crank chamber, located at the lower side of the engine lower.
According to another known art, the engine has a positive crankcase ventilation system (hereinafter referred to as “PCV system”), in which the oil separator separates oil from blow-by gas to return the oil to the oil pan through the oil discharge pipe. Oil or vaporized oil is separated from blow-by gas by the oil separator. While the oil returns to the oil reservoir in the oil pan through the oil discharge pipe, any oil remaining in blow-by gas flows to the engine intake system.
Intake-manifold vacuum acts on the oil separator in the PCV system in order to return blow-by gas collected from the crank chamber to the intake system. This results in lower pressure in the oil separator than in the crank chamber and the cam chamber. Thus, airflow is generated that flows into the oil separator through the oil discharge pipe. Accordingly, this airflow may cause blow-by gas containing a large amount of vaporized oil to flow into the intake system without through the oil separating mechanism provided in the oil separator. Therefore, in order to prevent such backflow of blow-by gas, preferably the downstream end of the oil discharge pipe is submerged in oil constantly.
FIG. 11 shows an example of the related-art oil pan structure. FIG. 11 is a sectional view illustrating a part of an oil pan 111 located below the engine crankcase. The oil pan 111 has a two-part structure, in which the oil pan 111 is divided into an upper case 112 and a lower case 113. The upper case 112 has an oil reservoir 114 on its one end. An oil discharge pipe 115 is provided in a crankcase to return oil from the upper part of the engine into the oil pan 111. The oil discharge pipe 115 has a downstream end located in the oil reservoir 114. The downstream end 115a of the oil discharge pipe 115 remains submerged in oil in the oil reservoir 114. In addition, the oil reservoir 114 has an oil drain hole 114a on its side surface 114b, so that oil in the oil reservoir 114 flows out through the oil drain hole 114a into the oil pan 111.
An example of the oil return structure is shown in JP-U-Hei 1-134714. This oil return structure is designed to prevent backflow of blow-by gas, while preventing oil from flowing out of the oil pan due to dislodgement of the oil discharge pipe. More specifically, the oil return structure, disclosed in JP-U-Hei 1-134714, is designed to return oil, separated from blow-by gas, to the oil pan through the oil return passage (oil discharge pipe). In the oil pan, the bottom section of the oil return passage is formed into a U-shape with its one end opened upward. The open end is located at a height between low oil level and high oil level in the oil pan. At the low oil level, a minimum allowable oil amount is ensured. At the high oil level, as well as a maximum allowable oil amount at the full level, an amount of additional oil to be absorbed by the lubricated parts are also ensured. In addition, the height from the bottom-most part of the U-shaped section to the open end is specified at a given height or greater such that no blow-by gas flows beyond the bottom-most part.
In the oil pan structure shown in FIG. 11, the side surface 114b of the oil reservoir 114 is located outward of the inner wall 113a of the lower case 113 (inner wall of the bottom part of the oil pan 111). This limits the location of the oil drain hole 114a to the side surface 114b of the oil reservoir 114. Thus, the only way to form the oil drain hole 114a is to obliquely drill the side surface 114b of the oil reservoir 114 from above. In addition, drilling limits the cross sectional shape of the oil drain hole 114a to circle. Also, total machining costs disadvantageously increases due to additional drilling. Further, oil at the bottom of the oil reservoir 114 flows less easily out of the oil drain hole 114a. This results in another disadvantage of sludge built up in the oil reservoir 114.
In turn, the engine oil return structure, disclosed in JP-U-Hei 1-134714, has the following problems. The U-shaped bottom section of the oil return passage has the upwardly open end. Thus, when the engine runs, oil flows through the oil return passage and returns to the oil pan through the open end of the U-shaped bottom section. In contrast, sludge flowing out through the oil return passage does not flow upward to the open end, but is deposited on the bottom-most part of the U-shaped section. In addition, the oil return passage need be long enough to extend from the upper part of the engine to the interior of the oil pan. In view of this, the longer oil return passage is more likely to interfere with components in the crankcase, and to avoid the interference, the oil return passage need be provided outside the crankcase.